If its loosening, wouldn't some sort of lock washer, or locktite thread locker be better than simply adding torque?

Yes. If the bolts were backing-off Loctite should help this (God, I hate disassembling with Loctite). But if it's bolt stretch, the only things that will solve it are higher grade bolts (which at this point only leaves 12.9) or a larger diameter bolt (but it needs to thread into the strut...)

Will other strut mounts with larger strut bolts fit this strut? I don't think anyone really wants to drill into their frame for a fix that might work, though...